A typical drive assistance device mounted on a vehicle, such as a car navigation system, calculates the travel time taken to travel from the present location of the vehicle to a destination as well as the shortest route to the destination by using traffic information, which is provided from the Vehicle Information and Communication System (VICS). VICS, which is used for drive assistance, collects various types of traffic information, such as congestion, travel time for different sections, and traffic restrictions, at a management center, referred to as the VICS center, and distributes the traffic information to drive assistance devices via communication media, such as radio beacons, infrared beacons, and FM multiplex broadcasting. Based on the received traffic information, the drive assistance device selects a recommended route from the present location to the destination and notifies the driver of the selected route.
The drive assistance device calculates a recommended route by referring to link costs, which are set for “links”, that is, predetermined road sections separated by traffic lights or intersections. A link cost serves as an index indicating mobility in each link (or ease of traveling in each link). The link cost information includes, for example, average travel time and road width. The drive assistance device refers to the link costs set for each link in each route from the present location to the destination to select a route that minimizes the link costs. However, the link costs calculated within the same link may differ depending on the vehicle that is subject to measurement or the timing at which the travel time is measured. This is because vehicles stop and start traveling in accordance with changes in the states of the traffic lights, which turn red (stop) or green (go). In other words, the link cost may vary within the same link. The link costs may also vary due to changes in the amount of traffic and the occurrence of traffic accidents. The standard error of the link costs would be large when the link costs calculated within the same link vary greatly. This lowers the reliability of the calculated travel time and the selected recommended route, which are based on the link costs.
Accordingly, patent document 1 describes a method that obtains the standard deviation of link costs for each link, which indicates the variation of the link costs, by using past statistical values of the link costs for each link. The calculated standard deviation is used as a risk factor when calculating the recommended route. With this method, a route having a small risk factor is selected as a reliable route. However, the selected route having a small risk factor may not be the shortest route to the destination. The selected route having a small risk factor may be longer than necessary. Thus, this method does not necessarily select a preferable route in terms of practicability.
Patent document 2 describes a method that determines a difference in the travel time between adjacent links based on the standard deviations of the links. When determined that the difference is large in the travel time between the adjacent links, the travel times of the links is determined as being abnormal values and removed from the database. Thus, the database accumulates travel times from which abnormal values have been excluded. However, the travel times (link costs) can often vary greatly between adjacent links under actual road situations due to traffic information, such as vehicles stopping and starting at traffic lights. If such travel times are uniformly excluded, it is difficult to build the travel time accumulation database reflecting the actual traffic conditions.
Accordingly, patent document 3 describes a method that obtains an average value from the median value of the travel time for each link acquired from the VICS and the arithmetic means value of the travel time for each link based on probe information collected by vehicles. The average value is used as a link cost. This method uses the probe information collected from vehicles in addition to the traffic information obtained from the VICS and thereby improves the reliability of the calculated link cost.